The loading and unloading of goods from pallets is a common cause of back injury because operators must lift the load from increasingly low positions as the load level reduces with unloading.
Load elevators which comprise a load supporting platform which is elevated by an electric motor or by hydraulics are known but are not widely used. Such load elevators are expensive and rely on the operator to maintain the load at the correct level as it is unloaded. Thus, if the operator does not adjust the position of the pallet as the load is removed, unloading of the pallet may still result in back injuries.
The use of spring biased platform for the elevation and lowering of loads as they are unloaded or loaded has been proposed in the past. For example, U.S. Pat. Nos. 1,212,341 Fowler and 2,980,287 and 3,091,503 Fisher shows spring biased mechanisms which are intended to maintain a loader at a convenient height as it is unloaded. However, the present inventors doubt that the device according to the Fowler disclosure would have ever achieved commercial realization, the springs being totally inadequate, or that the device as proposed by Fisher reached any application wider than the specific use mentioned in the specification, that is, spool troughs. None of the prior art disclosures address the question of how the springs are designed to take account of loads of varying height and weight, and as the present inventors discovered, the solution to this problem was not at all simple. It will be appreciated that the springs shown in the Fowler disclosure would simply not be capable of elevating the load of steel sheeting mentioned in the disclosure while the device as proposed by Fisher were only ever intended to be used with textile spools of a constant weight, size and number.
When the different nature of various loads is considered, it will be appreciated that the selection of the correct spring characteristics to maintain the top of the load at a relatively constant height as it is unloaded will not simply be a function of the weight of the load but also of its height. For example, a heavy dense load of the same weight as a load of lighter unit weight having a greater height will require completely different spring characteristics in order to cause elevation of the load in the correct manner as the load is unloaded.